1. Field of the Invention
The present invention relates to a wiring board for use in constituting a head substrate and the like of an LED (Light Emitting Diode) array head.
2. Description of the Related Art
A wiring board has been conventionally used for constituting a head substrate of an LED array head.
A conventional wiring board has a structure as shown, for example, in FIG. 3. A plurality of conductive layers 12 are deposited into a predetermined pattern on the top face of a substrate 11 made of a material such as glass and ceramics. In addition, the surface of each conductive layer 12 is covered with a plating layer 13.
As a material of the conductive layers 12, a conductive material containing metal such as silver (Ag) or aluminum (Al) is used. The conductive layers 12 are formed in the following process. Silver powder, glass frit, organic flux and so on are mixed with each other to obtain a predetermined conductive paste. The resultant conductive paste is printed and applied into a predetermined pattern on the top face of the substrate 11 by a conventionally known method such as screen printing. Then, the conductive paste is baked at a temperature ranging from 350 to 800xc2x0 C. to form the conductive layers 12.
The plating layer 13, which covers each of the conductive layers 12, is formed in order to achieve excellent bonding property at the time when an external electric circuit such as a semiconductor element and a flexible wiring board is bonded to the conductive layer 12, as well as to prevent the conductive layer 12 from corrosion. The plating layer 13 has been formed by a conventionally known method such as electric field plating where metal excellent in corrosion resistance such as nickel (Ni), gold (Au), chromium (Cr) or the like is selectively deposited on the surface of the conductive layer 12. When the plating layer 13 is formed by the method such as described above, the end portion of the plating layer 13 is extended to reach the top face of the substrate 11.
The above-described conventional wiring board, however, has the following problem. Whereas the end portion of the plating layer 13 is extended to reach the surface of the substrate 11 in such a manner that the plating layer 13 completely covers the entire conductive layer 12, the plating layer 13 is poor in adhesiveness to the substrate 11 in many cases. Due to this disadvantage, in the structure where a plurality of conductive layers 12 such as described above are arranged in parallel to each other at high density at intervals ranging from 10 to 100 xcexcm, large difference in potentials is created between the adjacent conductive layers 12, 12 when electric power is selectively applied to each of the plurality of conductive layers 12. As a result, the metal such as silver and aluminum which forms the conductive layer 12 may cause migration growing in the form of tree toward the conductive layer 12 located at a positive potential side via a space between the substrate 11 and the plating layer 13. In this case, the adjacent conductive layers 12 are electrically short-circuited, so that it is impossible to use the wiring board having such a trouble.
The present invention has been made to solve the above-mentioned problems, and an objective thereof is to provide a wiring board with high reliability capable of effectively preventing the occurrence of migration.
A wiring board in accordance with an aspect of the present invent includes: a substrate; a glass layer deposited on a top face of the substrate; a plurality of conductive layers made of conductive material including silver or aluminum and deposited on a surface of the glass layer; and a plating layer made of at least one of metals selected from a group consisting of nickel, chromium, and gold and covering at least a part of a surface of each conductive layer, wherein the glass layer is partially protruded so as to surround and to overlap both sides of each conductive layer, and a top end of each protruded portion is interposed between the conductive layer and the plating layer.
By such the configuration, the edges of each conductive layer are overlapped with the protruded portions of the glass layer located under the conductive layer. In addition, the top end of each protruded portion is interposed between the conductive layer and the plating layer formed on the conductive layer. Thus, even when a large difference in potentials is created between the adjacent conductive layers while the wiring board is used, the protruded portion of the glass layer sufficiently prevents the growth of the metal such as silver or aluminum which forms the conductive layer toward the adjacent conductive layer, and effectively prevents the occurrence of migration. As a result, electric insulation between adjacent conductive layers can be assuredly maintained over a long period of time, and reliability of the wiring board can be increased.